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Methane on Titan and Enceladus: Nature vs. Nurture?

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Methane on Titan and Enceladus: Nature vs. Nurture?

Dec. 07, 2009

Direct sampling of Titan’s atmosphere and Enceladus’ plume have provided key measurements for determining the original recipes for those moons. A big question has been whether these two moons were born with methane as part of their chemical make-up or whether methane was manufactured in Titan and Enceladus over time. The findings also provide greater understanding of the materials that were present in the nebula from which Saturn itself formed.

On Oct. 5, 2008, just after coming within 25 kilometers (15.6 miles) of the surface of Enceladus, Cassini captured this stunning mosaic as the spacecraft sped away from this geologically active moon of Saturn.

Over the course of its mission at Saturn, the Cassini Saturn orbiter and its Huygens probe have captured samples of atmospheric gases from the moon Titan and the water vapor-dominated plumes shooting into space from the moon Enceladus. These two very different moons of Saturn are slowly giving up secrets about how they formed, and letting us in on the original ingredients that went into creating the planets and moons of the solar system as a whole.

Recent research puts us closer to knowing the raw ingredients that formed the planets. Forming farther away from the sun, the bodies of the outer solar system didn’t undergo chemical changes from heating to the extent that Earth and its neighbors did when they formed in a hotter, denser part of the planet-forming cloud close to the young sun. The original recipe for the planets is better preserved in the comparatively undercooked worlds around Saturn.

Recent analysis of Cassini chemical data on Titan and Enceladus indicates that methane, in fairly large quantities, was one of those original ingredients in the solar nebula. The methane observed on Titan and Enceladus today appears to be part of the embryonic material, provided by the disk of gas and dust that formed the planets that went into making the moons when they first formed as bits of rock and ices called planetesimals. The data indicate that most of the methane in and on these moons is part of their original make-up, rather than newly produced through the interaction of rock, carbon dioxide and water deep in their interiors. These objects formed from solids that were initially produced in the solar nebula from which all the planets formed, but were then modified in the environment around Saturn during its own formation. These and other Cassini data are powerful in helping scientists constrain the steps and timing of planet and moon formation, and the role Saturn itself played in the birth of its moons.

This image is a composite of several images taken during two separate Titan flybys on Oct. 9 (T19) and Oct. 25 (T20) 2006.

In other recent work on Titan’s methane, scientists have modeled the process by which methane gas moves from the interior to the atmosphere and determined the time it takes for the methane to be lost to space. It turns out that about 60 to 180 million years ago, Titan could have had up to 3 to 4 times the amount of methane now observable there. That period, covering a time when large dinosaurs either flourished or were breathing their last on Earth, is believed to have been the start of the most recent period of major methane outgassing on Titan, when stores of methane gas in Titan’s interior began to be released to the surface. That is about the time when the methane seen on Titan today started to take center stage in Titan’s modern environment, shaping the landscape into the hazy, dune- and lake-dotted world Cassini is revealing to us today. Whether Titan had a constant or intermittent supply of methane in its much more distant past remains an outstanding question.

Central to these findings are data Cassini collected on the ratio of heavy hydrogen, or deuterium, to ordinary hydrogen in both Titan’s atmosphere and Enceladus’ plume. Variations of the deuterium-to-hydrogen ratio in organic compounds, and in water, provide clues to a planet or moon’s evolution, including the loss of gases that contain hydrogen.

Data used in the study came from the Cassini Ion and Neutral Mass Spectrometer and the Composite Infrared Spectrometer along with data returned in 2005 from the Huygens Probe’s gas chromatograph-mass spectrometer as it descended via parachute to Titan’s surface. Five of Cassini’s flights through the plume of Enceladus provided measurements of the same kind of isotope ratios.

As scientists discover more about the processes that led to the formation of bodies throughout our solar system, they can better understand how the Earth formed—and the likelihood that planets akin to Earth might form elsewhere in the cosmos.

Plume of ice particles erupting from Enceladus' south polar region.

This Cassini Science League entry is an overview of a science paper authored, or co-authored, by at least one Cassini scientist. The information above was derived from the following publications: